1. Field of the Invention
This invention relates to the field of manufacturing processes for articles of polymeric composition. More particularly, this invention relates to injection molding methods for the manufacture of expanded thermoplastic articles.
2. DESCRIPTION OF THE PRIOR ART
Injection molding of expanded thermoplastics is becoming more commercially feasable as labor and other manufacturing costs continue to rise. The low unit cost of injection molding may be combined with the improved physical properties of recently developed expanded thermoplastics for the production of sturdy articles of low weight, acceptable physical properties, and quite importantly, low manufacturing cost. These expanded articles such as appliance housings, furniture components, objects d'art, etc., are generally in the form of a rigid foamed thermoplastic structure covered by a non-porous and non-foamed skin of the same composition; the thickness of the skin ranging from a few mils to one-eighth of an inch or more.
Characterizing these injection molded expanded thermoplastic articles is a poor quality skin; the defects comprise in the majority swirls, visible flow lines, and mottled colors, as well as pin holes, cracks and fissures. These imperfections are primarily caused by the rapid cooling or chilling of the thermoplastic as it comes into contact with the mold surface in the cavity. The incomplete flow and expansion of these contacting portions of the mold charge are caught and then rapidly solidified. Most molders paint or otherwise mask these surface imperfections. The practice of heating the mold to allow the injected thermoplastic to continue its flow into the mold and complete its expansion prior to chilling or solidification has been found unacceptable as the increased temperature of the mold brings about an exponential increase in the mold cycle (dwell time) that very quickly lowers production rates and raises manufacturing costs to an uneconomical level. Another attempted solution has been the design and use of telescopic molds that utilize a first injection of a skinning material into a telescopically constricted mold cavity so as to form a skin thereover and then a second injection of expandable material into the interior of the first shot with a simultaneous telescopic expansion of the mold cavity to permit the expanding thermoplastic to drive the skinning material outward into contact with the mold surface and form a article having a good skin with an interior foam structure. Such a mold, however, is extremely expensive, and the molding machine is required to accept two completely different polymer compositions.
This invention is a method of injection molding expanded thermoplastic articles that may be practiced in conventional injection molding machines with only a minor amount of modification of the injection mold and may be practiced using cycle times quite near those of conventional injection molding cycles. The articles produced by this invention is characterized by a smooth, evenly colored surface over a substantial portion of the article.
Therefore, the main object of this invention is a method of producing expanded thermoplastic articles by injection molding techniques wherein the article is characterized by having a smooth skin of uniform color over a substantial portion of its surface. Other objects include a method of improving the surface of expanded thermoplastic articles using conventional mold techniques without resorting to expensive telescopic molds; a method of upgrading conventional injection molding techniques for the production of expanded thermoplastic articles; a method of producing expanded thermoplastic articles having a high quality skin and surface coloring without resort to secondary formulations, a method of reducing the time normally required to produce an injection molded expanded thermoplastic of high surface quality, a method of eliminating the requirement for post finishing of injection molded thermoplastic articles, and a method that is relatively easy to practice and that is amenable to automatic and semi-automatic process control. These and other objects of the invention will become more apparent upon a reading of the Description of the Preferred Embodiment in combination with the drawings attached hereto.